1. Field of the Invention
The invention relates to an optical instrument for measuring the speed or length of a moving surface, in which a measuring light beam from a laser is directed onto the surface and the Doppler-shifted stray light reflected by the surface and other light, e.g. non-Doppler-shifted laser light or some other Doppler-shifted laser light are superimposed for obtaining a beat, whose frequency forms a measure for the speed of the surface and is received and evaluated in an evaluation system in the form of continuously occuring digital signals, especially counted in a counter.
The accuracy of the measurement is inter alia dependent on the counting accuracy, i.e. on whether the occuring signals are continuously counted in a loss-free manner and the count results are also continuously supplied to the parts of the evaluation system following the counting system.
2. Prior Art
As the signal corresponding to the input frequency F.sub.in occur in a high measuring frequency range, the counting frequency is correspondingly high and can easily reach 10 MHz. If the counting system is in TTL technology, the use of a plurality of TTL components is necessary in view of the high counting frequency and these take up an undesirably large amount of space. Other solutions in discreet logic with low integration level are excluded due to the high counting frequency. It is also not possible to use a lower measuring frequency in the counting system, because the result cannot be in accordance with the required counting accuracy.